A microphone for collecting a sound and converting it into an electric signal is roughly divided into two, namely, a unidirectional one and an omni directional one. As compared to the omni directional microphone, the unidirectional microphone can collect the sound from the sound source located in a direction to which the microphone is directed with higher sensitivity than the case of collecting the sound from the sound source (obstructive sound source) located in other direction.
However, since one microphone has limitations in improving the directionality, in order to improve the directionality more, it has been considered to use a microphone array in which a plurality of microphones are arranged in a row (for example, refer to “Acoustic System and Digital Processing”, Institute of Electronics, Information and Communication Engineers, 1995, TOSHIAKI Ohga and others). A delay sum array as a typical system of the microphone array utilizes the fact that arrival times of the sounds from respective sound sources to respective microphones are different depending on spacious arrangements of respective microphones. Correcting arrival time differences of the sounds from the sound sources that are objects of recording to respective microphones and taking the average of the sound signals that are acquired from respective microphones, the sounds arriving from the sound sources that are objects of recording are emphasized and delete the sounds arriving from the directions other than these sound sources.
In addition, by automatically learning a filter which makes the sensitivity of the position of the obstructive sound source minimum, an adaptive beam former system as other system of the microphone array intends to selectively record only a sound from a sound source that is an object of recording.
There is also a system to estimate a position of a sound source by collecting the sound while moving the microphone (refer to Japanese Patent Application Laid-Open No. 8-292252).
According to the above-described delay sum array, considering a sound of a certain frequency, when the arrival time interval of the sound from the obstructive sound source to each microphone coincides with a time/an interval corresponding to one cycle of that frequency, according to the above-described average processing, the sound from the obstructive sound source is emphasized as same as the sound from the sound source that is an object of recording and this involves a problem such that an effect of separating the sound sources cannot be obtained. Specifically, in the case of recording the sound from a front direction of the microphone array as an object, there is a problem such that the sound of a certain frequency, which arrived from a certain direction and is not an object of recording, is recorded without being suppressed. This phenomenon is called as spacious aliasing.
In the adaptive beam former system, the number of the position where the sensitivity can be set at the minimum is limited to the number that one is subtracted from the number of the used microphones, and this results in that a capability of sound separation is lowered under the environment where many obstructive sound sources exist. In addition, it takes a certain period for learning of the filter and this involves a problem such that the capability of sound separation is lowered under the environment where the obstructive sound source is moving every moment. This is also a kind of spacious aliasing.
According to a method to collect the sound while moving the microphone in parallel on a rail described in Japanese Patent Application Laid-Open No. 8-292252, when the obstructive sound sources are separated, variation in a direction of the obstructive sound source due to movement in parallel is decreased. Therefore, there is a problem of the spacious aliasing yet.
Further, a capability of sound separation of the microphone array is decided by the number and arrangement of the microphone. In order to realize a high capability of sound separation, many microphones are necessarily used and this leads to a problem such that a cost is made higher and a space for setting cannot be managed.